Definition
Rendering is the computing process that turns scene data stored only as numbers (mostly a 3D model plus lighting information) into a final image or video you can actually see. You could say it puts flesh on an invisible skeleton of math, switches on the lights, and finishes the picture on screen.
Turning a blueprint into a photo-real picture
Looking at a flat blueprint, it's hard to imagine what a building will really look like. But when a skilled artist paints the rough texture of the bricks, the angle of the sunlight, and even the shadows and reflections in the windows, a stunning, photo-like rendering of the building appears.
The same thing happens in computer graphics. Inside the computer, a character's height and shape exist only as numbers: 3D coordinates for points and lines. On top of that, there are many physical rules that decide what color to apply, whether a surface shines like metal, and where the shadows fall behind it.
Gathering all that information and calculating light and texture to finish one complete picture is exactly what rendering is. Only after rendering can we finally see a lively 3D character moving on the screen.
Real-time rendering vs. pre-rendering
Rendering comes in two main flavors. One is "real-time rendering," which draws the screen nonstop so it can react instantly to what you do, like in a 3D video game. The graphics card redraws the screen dozens of times per second (typically 30 to 60, and even more on high-refresh-rate displays), so when your character turns their head, you see it right away.
On the other hand, 3D animated theatrical films and movies with special effects use "pre-rendering" (also called offline rendering). Since the audience doesn't need to control anything in real time, a render farm, a big cluster of computers, can spend hours on a single frame, and months on the whole project.
The trade-off for that extra time is that it can push realism and detail to the extreme, down to the sway of a single strand of fur or light reflecting off a water droplet. Games prize instant response, while movies prize overwhelming polish.
A bit more precisely: what the computer is calculating
To be more precise, rendering isn't just coloring things in. It's complex math. For realistic scenes in particular, the computer works out the path of light: how it leaves a lamp, bounces off the floor, reflects off a wall, and enters our eye (the camera). In practice, the calculation usually shoots rays backward from the camera and traces the path the light came from.
Ray tracing, which follows light paths one by one, and shader calculations, which apply surface textures, take trillions of math operations. In real-time rendering like games, the key is the parallel processing power of the graphics card (GPU), which runs thousands of calculation units at once. For pre-rendering in film, render farms that link together lots of CPUs (central processing units) are also widely used alongside GPUs.
Lighting techniques that used to be seen only in movies now run in real time on the console games and smartphone screens you enjoy, thanks to advances in high-performance hardware.
Common misconceptions
Rendering is just recording the finished screen or saving it as a video file.
Compressing and saving the picture to a file is closer to encoding. Rendering is the process of newly calculating the color of every single pixel from data such as a 3D scene or editing effects. In video editing software, the term "rendering" is sometimes used to cover both this calculation and the encoding.
Where you meet it
When a 3D animated movie is made, a render farm of many linked computers calculates each high-quality frame one at a time, and finishing the whole film can take months.
When you play a 3D game, the graphics card recalculates the screen dozens of times per second, so when your character looks around, you see the change right away.
In one sentence
Rendering is the process of taking the numbers and blueprints inside a computer and calculating light, shadow, and texture to draw a vivid picture people can see.